Electrowetting on dielectric (EWOD) is a well-known technique for manipulating droplets of fluid by the application of an electric field. Active Matrix EWOD (AM-EWOD) refers to implementation of EWOD in an active matrix array incorporating transistors, for example by using thin film transistors (TFTs). It is thus a candidate technology for digital microfluidics for lab-on-a-chip technology. An introduction to the basic principles of the technology can be found in “Digital microfluidics: is a true lab-on-a-chip possible?”, R. B. Fair, Microfluid Nanofluid (2007) 3:245-281).
FIG. 1 shows a part of a conventional EWOD device in cross section. The device includes a lower substrate 10, the uppermost layer of which is formed from a conductive material which is patterned so that a plurality of array element electrodes 12 (e.g., 12A and 12B in FIG. 1) are realized. The electrode of a given array element may be termed the element electrode 12. A liquid droplet 14, including a polar material (which is commonly also aqueous and/or ionic), is constrained in a plane between the lower substrate 10 and a top substrate 16. A suitable gap between the two substrates may be realized by means of a spacer 18, and a non-polar surround fluid 20 (e.g. oil) may be used to occupy the volume not occupied by the liquid droplet 14. An insulator layer 22 disposed upon the lower substrate 10 separates the conductive element electrodes 12A, 12B from a first hydrophobic coating 24 upon which the liquid droplet 14 sits with a contact angle 26 represented by θ. The hydrophobic coating is formed from a hydrophobic material (commonly, but not necessarily, a fluoropolymer).
On the top substrate 16 is a second hydrophobic coating 28 with which the liquid droplet 14 may come into contact. Interposed between the top substrate 16 and the second hydrophobic coating 28 is a reference electrode 30.
The contact angle θ is defined as shown in FIG. 1, and is determined by the balancing of the surface tension components between the solid-to liquid (γSL), the liquid-to non-polar surrounding fluid (γLG) and the solid to non-polar surrounding fluid (γSG) interfaces, and in the case where no voltages are applied satisfies Young's law, the equation being given by:
                              cos          ⁢                                          ⁢          θ                =                                            γ              SG                        -                          γ              SL                                            γ            LG                                              (                  equation          ⁢                                          ⁢          1                )            
In operation, voltages termed the EW drive voltages, (e.g. VT, V0 and V00 in FIG. 1) may be externally applied to different electrodes (e.g. reference electrode 30, element electrodes 12, 12A and 12B, respectively). The resulting electrical forces that are set up effectively control the hydrophobicity of the hydrophobic coating 24. By arranging for different EW drive voltages (e.g. V0 and V00) to be applied to different element electrodes (e.g. 12A and 12B), the liquid droplet 14 may be moved in the lateral plane between the two substrates 10 and 16.
Example configurations and operation of EWOD devices are described in the following. U.S. Pat. No. 6,911,132 (Pamula et al., issued Jun. 28, 2005) discloses a two dimensional EWOD array to control the position and movement of droplets in two dimensions. U.S. Pat. No. 6,565,727 (Shenderov, issued May 20, 2003) further discloses methods for other droplet operations including the splitting and merging of droplets, and the mixing together of droplets of different materials. U.S. Pat. No. 7,163,612 (Sterling et al., issued Jan. 16, 2007) describes how TFT based thin film electronics may be used to control the addressing of voltage pulses to an EWOD array by using circuit arrangements very similar to those employed in AM display technologies.
The approach of U.S. Pat. No. 7,163,612 may be termed “Active Matrix Electrowetting on Dielectric” (AM-EWOD). There are several advantages in using TFT based thin film electronics to control an EWOD array, namely:                Electronic driver circuits can be integrated onto the lower substrate 10.        TFT-based thin film electronics are well suited to the AM-EWOD application. They are cheap to produce so that relatively large substrate areas can be produced at relatively low cost.        TFTs fabricated in standard processes can be designed to operate at much higher voltages than transistors fabricated in standard CMOS processes. This is significant since many EWOD technologies require electro-wetting voltages in excess of 20V to be applied.        
EWOD droplet manipulation devices are a highly desirable platform for automation of chemical/biochemical reactions. Such devices may carry out chemical/biochemical reactions or reaction sequences in droplets that require complex droplet temperature profiles. Different steps of the reactions may need to be performed at different temperatures. There are many applications of EWOD devices that require the temperature of the sample/reagent droplets (and the products produced by combining them together) to be varied to facilitate the desired chemical or biochemical reaction. Many of these reaction protocols require droplets to be taken to multiple different temperatures at different times in the reaction sequence. Many reaction protocols require the droplets to be thermally cycled in time, in some cases undergoing many such thermal cycles. A significant example of a reaction protocol that requires precise temperature control in an EWOD device over many reaction cycles is droplet based nucleic acid amplification via polymerase chain reaction (PCR). PCR is a well-known reaction protocol for nucleic acid amplification.
One approach to handling multiple temperature requirements in an EWOD device is to provide multiple fixed temperature thermal zones. With such approach, conventionally a temperature control system, which usually is located external to the EWOD device, is arranged to control different parts of the EWOD device or “‘zones” to be at different and fixed temperatures. Accordingly, once heated to such temperature, the temperature remains constant in time. The temperature of droplets may then be modified by moving the droplets through the device to locations having different temperatures. This approach has been used particularly for EWOD devices constructed from a material having low thermal conductivity, such as for example glass, since it is possible to realize zones of different temperatures that are relatively close together in space, as heat is not transferred so easily laterally through the substrate material of the device. Each necessary temperature for a reaction protocol, therefore, must be provided in a separate thermal zone.
As examples of such approach, U.S. Pat. No. 9,452,433B2 (Shenderov et al., issued Sep. 27, 2016) describes a device for PCR amplification of nucleic acids comprising an EWOD device for droplet manipulation and two or more reaction zones with different temperatures, which are maintained at particular temperatures. A related patent in the same patent family, U.S. Pat. No. 9,517,469B2 (Shenderov et al., issued Dec. 13, 2016) describes related methods for PCR providing at least one reaction droplet to at least two reaction zones in the electrowetting array, each reaction zone having a different temperature needed for the nucleic acid amplification reaction and moving droplets using electrowetting between these reaction zones. Again, reaction zones with different temperatures are maintained at particular temperatures.
Another approach has been to adjust or vary device temperature in time. In such approach, the temperature of the EWOD device (or in an area of the device) can be varied in time, i.e. the temperature of the whole device or some substantial portion of the device can be modified.